Price is a big issue with SSDs. Another is capacity – that 240-GB SSD replaced a 320-GB traditional hard drive in MacBook Pro. Right now, you can buy a version of the Hitachi drive that came with my computer for under $90, though it would probably be less if not for Thai flooding that damaged many factories earlier this year. (By the way, hard disk prices are starting to recede.)

SSDs won’t gain mass acceptance until pricing comes down and capacity goes up. But there is an alternative that’s more affordable and provides almost the same performance – hybrid hard drives, which combine a small SSD with a disk-based drive. You get higher capacity and better performance at a fraction of the price of a pure SSD.

I’ve been testing Seagate’s 750-GB Momentus XT drive in my MacBook Pro. It includes a built-in, 8-GB SSD and a 7,200 RPM, 750-GB hard disk. The drive includes firmware that pulls frequently used data into the SSD, ready to be called up quickly when needed. It’s a similar concept to ReadyBoost, the Windows 7/Vista feature that uses a USB thumb drive or memory cardto speed up your PC.

In my tests, the Momentus XT was not quite as fast as an SSD, but was much faster than my original hard drive. It is a lot cheaper than an SSD – Amazon is currently selling it for $240. It has a list price of $350. It’s still significant more expensive than a traditional hard drive – 750-GB notebook drivesstart at under $100.

The Momentus XT installs like any other notebook drive, and will work in Macs or Windows PCs. After the initial setup, you’ll not see a big performance boost. But as you use the drive, it learns what data it needs to pull into its flash memory, and over time it becomes much faster.

For example, the SSD gave me a boot time of about 11 seconds. After just a couple of reboots with the Momentus XT, my MacBook Pro’s boot time was 15 seconds. Word 2011, notorious for taking a long time to launch, started up in 5 seconds on the SSD, and 6 seconds with the hybrid drive. Photoshop CS3 started in 5 seconds on the SSD, and 7 seconds on the Momentus XT.

I also saw great results with Windows 7 virtual machines running in Parallels 7. As with the SSD, the Momentus XT makes running Windows virtually as snappy as though it was running natively. It takes a little bit longer to resume from being suspended than the SSD, but only by a few seconds.

This is the second version of Seagate’s hybrid line. The original Momentus XT had problems with lockups and slowdowns over time that were eventually addressed with firmware upgrades. While I’ve had no problems at all, if you’re concerned it might be worth keeping an eye on Seagate’s customer forums for recurring issues. At the moment, though, I’d put my full confidence in this drive (so long as my Time Machine backups are up to date!).

The big question: Is this drive worth the premium you’ll pay over a traditional hard drive? If you’ve got the money to burn, I’d say yes. There’s also a 500-GB version that sells for $170 on Amazon, and that seems like a better deal. Traditional 500-GB notebook drives are starting now at under $100, making the premium for the smaller Momentus XT less painful.

Update: One thing I forgot to mention . . .

While the drive is very quiet, I can feel vibrations caused by its spinning when I have my hands on the notebook. My previous, traditional drive didn’t so this. It doesn’t seem to affect performance, and after a while I don’t notice the vibration, but I’d be curious if any other users of this drive have experienced this.